1. Field of the Invention
The present invention relates to an organic electroluminescent device or element (hereinafter, referred also to as an xe2x80x9corganic EL devicexe2x80x9d) which can be utilized as a planar light source or utilized in display devices, for example.
2. Description of the Related Art
Recently, attention has been drawn to organic EL devices in which a luminescent layer, i.e., light emission layer, is formed from a specific organic compound. The reason for the recent attention is that such organic EL devices can achieve a large area display device which can be operated at a low voltage. To obtain a highly efficient EL device, Tang et al., as is reported in Appl. Phys. Lett., 51, 913 (1987), have succeeded in attaining an EL device having a structure in which organic compound layers having different carrier transporting properties are laminated to thereby introduce holes and electrons with good balance via an anode and a cathode, respectively. In addition, since the thickness of the organic compound layers is less than or equal to 2,000 xc3x85, the EL device can exhibit a high luminance and efficiency sufficient for practical use; i.e., a luminance of about 1,0000 cd/m2 and an external quantum efficiency of about 1% at an applied voltage of no more than about 10 volts.
In this highly efficient EL device, Tang et al. have used magnesium (Mg) having a low work function in combination with an organic compound which is essentially considered to be an electrically insulating material, in order to reduce an energy barrier which can cause a problem during injection of electrons from a metal electrode. However, since the magnesium is unstable and is liable to oxidization, and also exhibits only a poor adhesion to a surface of the organic layers, magnesium was used after alloying. Alloying is carried out by vapor co-deposition or simultaneous evaporation of magnesium and silver (Ag) which is relatively stable and exhibits good adhesion to the surface of the organic layers.
The researchers of Toppan Printing Co. (cf, 51st periodical meeting, Society of Applied Physics, Preprint 28a-PB-4, p.1040) and researchers of Pioneer Co. (cf, 54th periodical meeting, Society of Applied Physics, Preprint 29p-ZC-15, p.1127) have had developments in the usage of lithium (Li), which has an even lower work function than that of Mg, and alloying Li with an aluminum (Al) to obtain a stabilized cathode, thereby embodying a lower driving voltage and a higher emitting luminance in comparison with those of the EL device using Mg alloy. In addition, as is reported in IEEE Trans. Electron Devices., 40, 1342 (1993), the inventors of the present application have found that a two-layered cathode produced by depositing lithium (Li) alone with a very small thickness of about 10 xc3x85 on an organic compound layer, followed by laminating a silver (Ag) to the deposited Li layer is effective to attain a low driving voltage in an EL device.
Recently, Pei et al. of Uniax Co. have proceeded to reduce the driving voltage of an EL device by doping a polymeric luminescent layer with a Li salt (cf. Science, 269, 1086 (1995)). This doping method is intended to dissociate the Li salt dispersed in the polymeric luminescent layer to distribute Li ions and counter ions near the cathode and near the anode, respectively, thus ensuring an in-situ doping of the polymer molecules positioned near the electrodes. According to this method, since the polymers near the cathode are reduced with Li as a donor dopant, i.e., electron-donating dopant, and thus the reduced polymers are contained in the state of radical anions, a barrier of the electron injection from the cathode can be considerably reduced, contrary to the similar method including no Li doping.
More recently, the inventors of the present application have found that the driving voltage of an EL device can be reduced by doping an alkali metal such as lithium and the like, an alkali earth metal such as strontium and the like, or a rare earth metal such as samarium and the like, to an organic layer adjacent to the cathode electrode (cf. SID 97, Digest, P.775). It was believed that such reduction of the driving voltage could be obtained because a barrier in the electron injection from the cathode electrode can be notably reduced due to a radical anion state in the organic layer adjacent to the electrode produced by metal doping therein.
However, due to oxidation of the electrodes and other reasons, deterioration of the device can result in the above-described EL devices using an alloy of Mg or Li as the electrode material. In addition, the use of such alloy electrodes has the disadvantage of limited selection of a material suitable for an electrode, because the electrode material to be used has to simultaneously satisfy the requirement for the function as a wiring material. Furthermore, the above described two-layered cathode developed by the present inventors is unable to act as a cathode when the thickness of the Li layer is more than about 20 xc3x85 (cf. IEEE Trans. Electron Devices., 40, 1342 (1993), and also has the disadvantage of low reproducibility, because there is difficulty in the control of the layer thickness when the Li layer is deposited at a considerably reduced thickness in the order of about 10 xc3x85. Furthermore, in the in-situ doping method developed by Pei et al. in which the Li salt is added to the luminescent layer to cause their dissociation in the electric field, there is a problem with the transfer time of the dissociated ions to the close vicinity of the electrodes having a controlled velocity, thereby causing a considerable retardation of the response speed of the devices.
Moreover, in the method which includes doping the metal as a dopant in the organic layer, it is necessary to precisely control the concentration of the dopant during formation of the organic layer, because the doping concentration may affect the properties of the resulting devices.
The present invention has been made to solve the above-described problems of the prior art EL devices, and accordingly, one object of the present invention is to reduce the energy barrier in the electron injection from a cathode electrode to an organic compound layer in accordance with a simple and reliable method to thereby ensure a low driving voltage of the EL devices regardless of the work function of the cathode material.
Another object of the present invention is to provide a device (organic EL device) capable of ensuring satisfactory characteristics which are similar to, or better than, those obtained using the above-described alloy as the electrode material, even if aluminum or other low-cost stable metals which are conventionally used as the wiring material in the prior art are used solely as the cathode material.
In order to achieve the above mentioned objects, an organic electroluminescent device is provided which includes at least one luminescent layer, constituted from an organic compound, between a cathode electrode and an anode electrode opposed to the cathode electrode. The electroluminescent device further includes an organic layer adjacent to the cathode electrode, the organic layer being a mixed layer of an electron-transporting organic compound and an organic metal complex compound containing at least one member selected from the group including an alkali metal ion, an alkali earth metal ion and a rare earth metal ion. The cathode electrode includes a metal capable of reducing the metal ion(s) in the organic metal complex compound of the mixed layer, in a vacuum, to the corresponding metal.
Preferably, the mixed layer is a layer formed upon co-deposition of the organic metal complex compound and the electron-transporting organic compound.
Preferably, the metal used in the formation of the cathode electrode is any one of aluminum, zirconium, titanium, yttrium, scandium and silicon.
Preferably, the metal used in the formation of the cathode electrode is an alloy containing at least one of aluminum, zirconium, titanium, yttrium, scandium and silicon.
The above-described cathode metals and metal alloys have a high melting point, and under a vacuum, can reduce a metal ion in the organic metal complex compound to the corresponding metal.
Generally, some alkali metals, alkali earth metals and rare earth metals can exhibit a higher saturated vapor pressure than that of high-melting-point metals such as aluminum, and therefore any compound containing such alkali metals, or the like, can be reduced with the high-melting-point metals such as aluminum, silicon, zirconium and the like. For example, it is well known that calcium oxide can be reduced with aluminum to form a liberated metal calcium (cf. Chemical Handbook, xe2x80x9cApplied Chemistry Section Ixe2x80x9d, edited by the Chemical Society of Japan, Maruzen Co., p.369), and rubidium oxide and strontium oxide (cf. Metal Handbook, edited by the Japan Institute of Metals, Maruzen Co., pp.88-89) can be also reduced with aluminum to form a liberated metal rubidium and strontium, respectively.
The production of metal electrodes in the organic EL devices is carried out in a vacuum of not more than 10xe2x88x925Torr to deposit an atomic metal on a substrate upon melting and volatilization of the metal. Therefore, when a thermally reducible metal such as aluminum, silicon, zirconium, and the like, in an atomic state is applied onto the alkali metal compound, alkali earth metal compound or rare earth metal compound, the above-described reduction reaction in vacuum results so as to produce a reduced and liberated metal from the corresponding metal compound. In this reduction process, if the electron injection layer is constituted from a mixed layer including the organic metal complex compound and the electron transporting organic compound; the alkali metal, alkali earth metal or rare earth metal produced upon reduction and liberation of the organic metal complex compound can effectively reduce the adjacent electron transporting organic compound, thereby forming a metal doping layer.
If the alkali metal, alkali earth metal or rare earth metal compound to be reduced is an inorganic compound such as an oxide, a fluoride, and the like, it is sometimes difficult to make co-deposition of such metals with the electron transporting organic compound to form an organic layer, because the inorganic compound has a high evaporation temperature due to good stability thereof. Furthermore, due to high electrical insulation property of the inorganic compound, the remaining molecules of the inorganic compound which are unreduced may increase the driving voltage of the EL device.
In the present invention, an alkali metal, alkali earth metal or rare earth metal compound was used as the organic metal complex compound, in place of the inorganic compound thereof. The organic metal complex compound and the electron transporting organic compound were co-deposited and mixed to form a mixed layer. The produced mixed layer was further coated with a cathode electrode made of a specific electrode material which contains a metal capable of reducing, in a vacuum, the metal ion(s) contained in the organic metal complex compound. Thus, based on the high reducing power of the cathode metal in a vacuum, a metal could be reduced and liberated from the organic metal complex compound in the mixed layer and then the adjacent electron transporting organic compound could be reduced with the liberated metal. The inventors of the present invention have thus succeeded to diminish an electron injection barrier, thereby reducing a driving voltage of the device.
In practice, the organic metal complex compound used in the formation of the electron injection layer is not restricted to a specific compound. However, the following is preferably used as a metal ion thereof: at least one metal ion of the alkali metal ions, alkali earth metal ions and rare earth metal ions. As the ligand compound for the metal complex compound, although they are not restricted to the below-described, quinolinol, benzoquinolinol, acrydinol, phenanethridinol, hydroxyphenyloxazole, hydroxyphenyl thiazole, hydroxydiaryloxadiazole, hydroxydiarylthiadiazole, hydroxyphenylpyridine, hydroxyphenylbenzoimidazole, hydroxybenzotriazole, hydroxyfurborane, bipyridyl, phenanethroline, phthalocyanine, porphyrin, cyclopentadiene, xcex2-diketones, azomethines and derivatives thereof, are preferably used.
The present disclosure relates to subject matter contained in Japanese Patent Application No.10-357899 (filed on Dec. 16, 1998) which is expressly incorporated herein by reference in its entirety.